You have no doubt been exposed many times to the Bohr model of the atom. You may have even learned of the connection between this model and bright line spectra emitted by excited gases. In this experiment, you will take a closer look at the relationship between the observed wavelengths in the hydrogen spectrum and the energies involved when electrons undergo transitions between energy levels.
The energy of a photon is related to its frequency by the equation E = hf, where h is Planck’s constant. By determining the potential required to excite an LED to emit light, you can estimate the energy of the photons emitted. Using a spectrometer, you can measure the peak wavelength of the emitted light; from this, the frequency can be calculated. Performing this analysis for a number of LEDs will enable you to obtain a reasonable approximation of the value of Planck’s constant.
Polarimeters can be used in kinetics experiments to follow the change in concentration of an optically active sample as a reaction proceeds. Sugars are common examples of optically active compounds. Sucrose is a disaccharide that can be broken down into its two substituent monosaccharides, glucose and fructose.
This process occurs too slowly in water to be monitored on any real time scale, so a catalyst, acid or enzyme, must be added to accelerate the reaction rate. In this experiment, hydrochloric acid is used to catalyze the reaction while its rate is monitored using a polarimeter. The experiment will be repeated using the enzyme invertase to catalyze the reaction.
One well-known application of gas chromatography is its analytical capability used to obtain purely physiochemical data such as activity coefficients of solutes in various solvents, heats of solution, and enthalpies of vaporization of volatile compounds. It can also be used to demonstrate colligative properties. Here, we introduce the determination of the enthalpy of vaporization using retention times measured with a gas chromatograph (GC).
Gas chromatography is based on a solute in a mixture partitioning itself between the mobile phase and the stationary phase. With the Vernier Mini GC, the mobile phase is air and the stationary phase is a nonpolar phase capillary column. The amount of time a given chemical spends in the stationary phase relative to the amount of time it spends in the mobile phase is a very important quantity in elution chromatography; it is called the capacity factor, k′, and is given by:
where tR is the retention time of the compound; that is, the amount of time the chemical spends in the column from the point of injection to the point of detection. The time it takes for the mobile phase to pass through the column is referred to as tM; it is typically the retention time of a non-retained species. In this experiment, the non-retained compound you will be using is acetone and it functions as a very important standard to help normalize the amount of time it takes a species to run through the column, enabling calculation of k′. As part of this calculation, we are assuming that the retention time of the non-retained species (acetone) is independent of temperature.
To relate the capacity factor to the enthalpy of vaporization, the following equation is used:
where ΔHvap is the standard enthalpy (heat) of vaporization of the compound. This value is assumed to be independent of temperature. T is the temperature in Kelvin, R is the gas constant in appropriate units, and C is a constant. The equation is written in the slope-intercept form where the value of ΔHvap is determined by plotting ln(k′/T)vs.1/T.
The fact that most of it [hardware] is used in the introductory chemistry classes speaks to the ruggedness and utility of the devices.
Ed Gray, Ph.D.West Hartford, Connecticut
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